Crystal growth and structural investigation of A2BReO 6 ( A=Sr, Ba; B=Li, Na)

Abstract

Single crystals of the double perovskite rhenates A 2 BReO 6 ( A=Sr, Ba; B=Li, Na) were grown out of molten hydroxide fluxes. Single crystals of orange/yellow Ba 2LiReO 6, Ba 2NaReO 6 and Sr 2LiReO 6 were solved in the cubic, Fm-3 m space group with a=8.1214(11) Å, 8.2975(3) Å, and 7.9071(15) Å, respectively, while Sr 2NaReO 6 was determined to be monoclinic P2 1 /n with a=5.6737(6) Å, b=5.7988(6) Å, c=8.0431(8) Å, and β=90.02(6) °. The cubic structure consists of a rock salt lattice of corner-shared ReO 6 and MO 6 ( M=Li, Na) octahedra which, in the monoclinic structure, are both tilted and rotated. A discrepancy exists between the symmetry of Sr 2LiReO 6 indicated by the single-crystal refinement of flux-grown crystals (cubic, Fm-3 m) and the symmetry indicated by the powder diffraction data collected on polycrystalline samples prepared by the ceramic method (tetragonal, I4/ m). It is possible that the cubic crystals are a kinetic product that forms in small quantities at low temperatures, while the powder represents the more stable polymorph that forms at higher reaction temperature.